1. Field of the Invention
The present invention relates to a method for fabricating a laser diode having an optical fiber Bragg grating as a front mirror of external resonator and a laser diode fabricated thereby, and more particularly to a method for fabricating a laser diode having an optical fiber Bragg grating as a front mirror of external resonator and a laser diode fabricated thereby, in which when an optical fiber having the optical fiber Bragg grating formed thereon is coupled to a TO-CAN packaged laser diode chip using a fiber pigtailing method, a thermal epoxy resin is applied to a designated part of the outer cylindrical surface of the optical fiber at a region, at which the optical fiber Bragg grating is not formed, is located in a ferrule, and is then cured so that variation in optical characteristics of the optical fiber Bragg grating, after the optical fiber having the optical fiber Bragg grating is mechanically fixed in the ferrule, is minimized, thereby using the optical fiber Bragg grating having a stabilized oscillation frequency as front mirror of the external resonator.
2. Description of the Related Art
Generally, a laser diode having an optical fiber Bragg grating as a front mirror of external resonator improves stability of an oscillation frequency due to temperature and injected current by placing the optical fiber Bragg grating serving as a front mirror at a light output terminal.
Since the external resonating laser diode using the above optical fiber Bragg grating as a front mirror of external resonator employs an optical fiber made of silica having excellent temperature stability, the laser diode has an excellent output frequency stability according to variation in injected current or external temperature compared to a Fabry-Perot laser, a Distributed Feedback (DFB) laser, and a Distributed Bragg Reflector (DBR) laser. Further, since the optical fiber Bragg grating is used as the front mirror of the external resonator, it is possible to embody lasers having various objects according to optical characteristics of the optical fiber Bragg grating and it is easy to select and control an oscillation frequency. Particularly, since the stability of the oscillation frequency is excellent, it is unnecessary to additionally control temperature, thereby reducing the production costs of the laser.
In fabrication of the laser using the above optical fiber Bragg grating as the front mirror of the external resonator, it is important to fabricate the laser without changing optical characteristics of the optical fiber Bragg grating serving as the front mirror of the external resonator. In order to form a optical coupling between optical fiber and a TO-CAN packaged laser diode module, a cylindrical free space located in a ferrule is filled with a thermal epoxy resin, the optical fiber is inserted into the ferrule, and the thermal epoxy resin is cured so that the optical fiber is fixed to the ferrule. FIGS. 1A and 1B illustrate a conventional process for fabricating the above laser.
With reference to FIG. 1A, in order to form optical coupling between optical fiber and a TO-CAN packaged laser diode chip, in the conventional process for fabricating the laser, an optical fiber 11 is fixedly inserted into a cylindrical-structured ferrule 12. In this process, a cylindrical free space is formed through the ferrule 12 along the longitudinal direction, and is filled with a thermal epoxy resin 13. When the optical fiber 11 is inserted into the ferrule 12 filled with the thermal epoxy resin 13, and is heated, the thermal epoxy resin 13 is cured, thereby fixing the optical fiber 11 to the ferrule 12 under the condition that the optical fiber 11 is inserted into the ferrule 12. Thereafter, as shown in FIG. 1B, under the condition that the optical fiber 11 is fixedly inserted into the ferrule 12, the ferrule 12, to which the optical fiber 11 is fixed, and a TO-CAN packaged laser diode chip 14 are optically coupled, thereby producing a laser.
In the above conventional method for fabricating the laser, since the cylindrical free space formed through the ferrule 12 is filled with the thermal epoxy resin 13 and then the optical fiber 11 is inserted into the ferrule 12, as shown in FIG. 1B, the overall outer cylindrical surface of the optical fiber 11 inserted into the ferrule 12 is fixed by the thermal epoxy resin 13. In the above conventional method, when the thermal epoxy resin 13 filling the cylindrical free space located in the ferrule 12 is cured, fine strain is applied to the optical fiber 11 inserted into the ferrule 12. The applied strain does not comparatively affect optical characteristics of a conventional single mode optical fiber, but changes reflectance spectrum characteristics of an optical fiber having an optical fiber Bragg grating having wavelength selectivity. FIGS. 2A and 2B are schematic views illustrating problems generated from the optical fiber having the Bragg grating formed thereon due to the conventional process for fabricating the laser.
As shown in FIG. 2A, a cylindrical free space formed through a ferrule 22 is filled with a thermal epoxy resin 23, and an optical fiber 21 having an optical fiber Bragg grating 211 formed thereon is inserted into the ferrule 22. A region of the optical fiber 21, at which the Bragg grating 211 is formed, is located in the ferrule 22, and the thermal epoxy resin 23 is located at the overall outer cylindrical surface of the optical fiber 21 inserted into the ferrule 22 according to the conventional method. In this state, when the optical fiber 21 is heated using a heater so as to be fixed to the ferrule 22, the thermal epoxy resin 23 is cured.
In the above process, as shown in FIG. 2B, the thermal epoxy resin 23 is cured by heating so that irregularly distributed thermal epoxy resin 23′ is obtained. Then, when the optical fiber 21 and the ferrule 22 are mechanically fixed to each other, the characteristics of optical fiber Bragg grating 211 is changed. That is, when the optical fiber 21 having the optical fiber Bragg grating 211 formed thereon is fixed to the cylindrical free space located in the ferrule 22, strain is applied to the optical fiber 21 due to the irregular curing of the thermal epoxy resin 23, thereby changing the Bragg grating 211 having uniform period in sub micrometer unit to the irregular fiber Bragg grating 211′ having uniform period. Then, the change of the period in the fiber Bragg grating has a negative effect on a reflection spectrum of the optical fiber Bragg grating.
That is, since the Bragg resonance frequency of the optical fiber Bragg grating is changed by the strain generated when the thermal epoxy resin is cured, an oscillation frequency of the fabricated laser is outputted from a frequency region differing from a designed frequency region. Particularly, when the reflectance of the optical fiber Bragg grating is changed, the resonance does not occur, thereby not generating optical output of the laser.